CINDY DAY: Flash, bang - but why the rumble?

Darrell Cole snapped this great photo of a roll cloud detached from the main thunderstorm cloud. It was taken last month over Amherst, N,S. Five to 10 minutes later, he saw several bolts of lightning, but no rain. - Darrell Cole

I love a good storm. Growing up, I was never afraid of thunder and lightning. I’m not sure how that happened; both mom and Grandma were!

Every summer, powerful storms rolled down the St. Lawrence valley region. In school, there were always a few kids who were frightened by the crashing of thunder. I remember Mrs. Flaro telling us there was nothing to fear… the Lord was bowling.

The other day I received an email from a man who wanted to know why thunder rumbles.

Thunder is caused by lightning, which is a stream of electrons flowing between clouds or between a cloud and the ground. The air surrounding the electron stream can get as hot as 27,000 C. How hot is that? It’s three times hotter than the surface of the sun!

The sudden heating causes the air to expand as the flash passes through the air and immediate cooling contracts the air again. As the air cools, it produces a resonating tube around the lightning’s path.

Now to the rumbling. Typically, a lightning bolt can be several kilometres long. Let’s say the nearest part of the bolt is one kilometre away. Sound travels about one km in three seconds so you will hear the first part – the clap of the lightning – three seconds after the flash. If the farthest part of the bolt is five km away, it will take 15 seconds to hear that part of the bolt. From three seconds to 15 seconds after the flash you will hear every different clap in between, resulting in a rumbling sound. The end of the rumble is the farthest part of the bolt.

Sometimes understanding something can make it a little less scary.

On a final note, did you know that nearly 1,800 thunderstorms are happening at any moment around the world? That's 16 million a year!